Communication systems, and particularly cellular radiotelephone systems, typically transfer communication of a subscriber unit from one cell to another by attempting to measure the signal strength, and perhaps other measures of communication quality, of a subscriber unit, or mobile. In cellular radiotelephone systems, the process of using quality measurements to choose a better communication path is known as handoff. As digital cellular radiotelephone systems mature, additional signal quality criteria, such as quality of the uplink and downlink path of the subscriber/base-station communication, can be implemented to aid in the handoff process. The process of measuring all the desired signal quality criteria, however, is time intensive, at least on the order of several seconds. When the signalling required to perform handoff, such as signalling between a switch and base-stations involved in the handoff, is included, additional seconds are added to the already time intensive task.
In certain cellular radiotelephone system regions, traffic handling capacity can only be increased by making the cell size smaller and smaller. As the shrink of cell size continues, eventually the traditional cell becomes a mini-cell, or micro-cell. The difference between normal cells and micro-cells is readily distinguishable. For example, a normal cell may be characterized by having its coverage area typically greater than one square mile; antennas elevated significantly above most nearby structures so that the resultant radiation pattern is primarily determined by the antenna itself; and good in-street signal strength within the required coverage area. Micro-cells, on the other hand, may be characterized by having a coverage area less than one square mile-usually much less; antennas below many of the neighboring structures so that the resultant radiation pattern is primarily determined by the nearby reflectors and not the antenna directivity; and good in-building signal strength within its coverage area. The coverage area is more or less determined by the regions of good signal strength. As usage increases and/or the need for in-building communication builds, more of the spectrum will be allocated to the micro-cells.
In micro-cellular systems, the variation in measured signal strength is even greater than that for larger, normal cell systems for a variety of propagation reasons. The greater variation in the mean value of the signal strength would require an even longer measurement time to establish the mean, and several reasons as to why this is unacceptable exist. First, the subscriber unit could be moving through the cells at a rate which would put the subscriber unit out of the cell by the time the measurement was made. Second, the expected rapid variation in the mean signal strength can suddenly cause the signal strength to drop signficantly below an acceptable level. This might occur when a subscriber unit simply turns a corner in an urban environment and its signal fades temporarily. These factors, when combined, actually serve to limit the minimum size a micro-cell can take on; cell size is typically measured by approximate cell radius or by the distance between base sites needed to serve an area. Since the ability of the system to serve a certain number of subscribers is directly proportional to the size of the cell, these factors, in traditional cellular system designs, directly limit the capacity of the system.
The use of micro-cellular systems also brings with it the inherent problem of co-channel interference. As in larger, normal cellular systems, the use of a reuse pattern in micro-cellular systems would help to mitigate or even eliminate co-channel interference. However, reuse patterns limit the overall subscriber capacity when viewed on a per-cell basis.
Thus, a need exists for a radiotelephone system incorporating micro-cellular structure which accommodates for rapid variations of signal quality criterion and mitigates the effects of co-channel interference while maintaining a higher subscriber capacity than traditional communication systems.